Compound semiconductor devices are widely used for optoelectronic applications. For example, compound semiconductors composed of materials from group III-V are best suitable for light-emitting diodes (LEDs). LEDs are manufactured by forming active regions on a substrate and by depositing various conductive and semiconductive layers on the substrate. The radiative recombination of electron-hole pairs can be used for the generation of electromagnetic radiation (e.g., light) by the electric current in a p-n junction. In a forward-biased p-n junction fabricated from a direct band gap material, such as gallium arsenide (GaAs) or gallium nitride (GaN), the recombination of the electron-hole pairs injected into the depletion region causes the emission of electromagnetic radiation. The electromagnetic radiation may be in the visible range or may be in a non-visible range. Different colors of LEDs may be created by using compound materials with different band gaps.
Crystalline compound semiconductor materials, such as GaN, are typically formed by epitaxially growing a compound semiconductor layer upon a crystalline substrate of another material, such as a sapphire substrate that has a matching crystallographic plane and is more easily formed. The GaN layer thus formed are processed into electronic or optoelectronic devices, such as LEDs, based upon the properties of GaN. The compound semiconductor devices are then detached from their growth substrate and reattached to other semiconductor or non-semiconductor substrates to be integrated with other electronic components for the intended applications.
Various techniques exist in separating a compound semiconductor layer from a growth substrate. In one attempt, an epitaxial sacrificial layer is first grown on the substrate. The compound semiconductor layer is then epitaxially grown on the sacrificial layer. After the compound semiconductor layer is processed with the intended devices, it is separated from its growth substrate by a wet etching process, which selectively etches away the sacrificial layer, thereby lifting off the compound semiconductor layer. The free-standing compound semiconductor film may be then bonded to other substrates. The compound thin film may be further processed to integrate the functionalities of the compound semiconductor devices and of devices in the other substrate material.
The above existing separating process relies upon a liquid etchant dissolving from the sides of a very thin sacrificial layer between the growth substrate and epitaxially formed compound semiconductor film. The separating process can be very time-consuming, especially for separating large area films, and are economically unfavorable for manufacturing processes of large scale.
In another attempt, an optical process is deployed in lifting off compound films from a growth substrate. As an example, a GaN film is epitaxially grown on a sapphire substrate. The resultant structure is then irradiated from the sapphire side with an intense laser beam. This wavelength of the laser is within the sapphire bandgap so that the radiation passes through it, but the irradiation wavelength is somewhat outside of the absorption edge of GaN. As a result, a significant portion of the laser energy is absorbed in the GaN next to the interface. The intense heating of the GaN separates the gallium from gaseous nitrogen, thereby separating the GaN thin film from the sapphire substrate.
The process, however, suffers various difficulties. As an example, the high energy laser radiation may blow away the overlying GaN film, and fracturing of the GaN film often occurs. Moreover, the area of the high-energy laser beams is limited, which makes separating large area films difficult.